Seems that the latest update of Markdown Editor v2.0.6 has broken the app. Mixed and Preview modes no longer work. Have created a Issue with the developer.
I saw another issue from another user probably on GitHub already.
The solution was to clear the browser cache once and reload the page of your NC server:
I am using the same version and the app works for me.
Please try out the browser cache deletion and if that works, update your issue on Github accordingly.
Please add link to your issue here.